This invention relates to a heat-sensitive record material and particularly to a heat-sensitive record material in which both of the sliding property and the retainability of recorded images are improved without accompanying either lowering the initial color density of recorded images or sticking.
There has been well known heat-sensitive record materials utilizing the colorforming reaction between a basic colorless chromogenic material and an electron accepting acidic reactant material, in which color images are produced by heating to contact with each other of the basic colorless chromogenic material and the electron accepting acidic reactant material. Recently, recording systems of the heat-sensitive record materials have been very rapidly developed so that the recording speed becomes higher and the recording machine becomes compact and energy saving.
Resultantly, the applied field has become very wide. The properties required for the heat-sensitive record materials are increased. Particularly, in the case of using recording machines which are energy saving, such as small facsimiles or small printers, it is required for the heat-sensitive record material to have high sensitivity and good sliding property. The "good sliding property" means that the heat-seisitive record material is smoothly passed through recording machines with low friction.
Further, general heat-sensitive record materials have such a disadvantage that, contacting the recording layer with fats and oils or a plastic film, color images are easily discolored or faded by the effect of fats and oils or the plasticizer comprised in the plastic film.
Accordingly, it has been strongly desired to provide a heat-sensitive record material which has good sliding property and which can produce color images having a high color density and being superior in color-retainability without discoloration or fading by the effect of fats and oils or a plasticizer.
It has been well known that a metal soap or a wax is added to the coating layer applied on general record materials to improve the sliding property. In he case of heat-sensitive record materials, there is described in Japanese Patent Publication No. 14531 of 1975, U.S. Pat. No. 3,988,501 and U.S. Pat. No. 4,032,690 that the sliding property is improved by adding waxes to the recording layer to reduce the frictional heat generated by rubbing, scratching and the like and to prevent unnecessary color developing, namely "color developing by scratching", in the recording layer.
However, in order to improve the sliding property enough to satisfy such recent requirement as described hereinbefore, it is necessary to use metal soaps or waxes in a very large amount so that the initial color density of recorded images is remarkably reduced and it can not be prevented to adhere smudges on the thermal head. With the use of some modifiers for improving the sliding property, there is accompanied the problem "sticking", that is, the thermal head sticking to the recording layer. Further, since the heat-sensitive recording layer is formed as the most outer layer, the product becomes a heat-sensitive record material very poor in the retainability of recorded images.
U.S. Pat. No. 4,554,566 discloses a heat-sensitive record material having a recording layer in which fine particles of a material selected from the group consisting of ethylene-.alpha.-olefin copolymer, lower density polyethylene and ethylene-vinyl acetate copolymer, having an average particle size of 0.03 to 16 microns, are added within the range of 5 to 50% by weight. The heat-sensitive record material is poor in the retainability of recorded images.
On the other hand, U.S. Pat. No. 4,370,370 discloses forming a protective layer comprising a water-soluble polymer on the recording layer to improve the retainability of recorded images.
However, even though, in order to improve the sliding property of heat-sensitive record materials having a protective layer, waxes are added to the protective layer, there has not been obtained a heat-sensitive record material which has a good sliding property and is superior in both of the initial color density and the retainability of recorded images without sticking.
The object of the invention is to provide heat-sensitive record materials in which the problems such as adhering smudges to the thermal head and sticking are practically prevented and which are superior in sliding property, initial color density and retainability of the recorded images.